Various glucose test strips have been described in the patent literature, such as, for example, JP5-72172 (Mar. 23, 1993; JP11-304748 (May 11, 1999); WO 01/25775; WO99/13099; EP1152239A1; WO02/00918A3; and WO2005/047528. One example of such test strips include electrochemical glucose test strips, such as those used in the OneTouch® Ultra® whole blood testing kit, which is available from LifeScan, Inc., are designed to measure the concentration of glucose in a blood sample from patients with diabetes. The measurement of glucose is based upon the specific oxidation of glucose by the enzyme glucose oxidase (GO). The reactions which may occur in a glucose test strip are summarized below in Equations 1 and 2.Glucose+GO(ox)→Gluconic Acid+GO(red)   Eq. 1GO(red)+2Fe(CN)63−→GO(ox)+2Fe(CN)64−  Eq. 2
As illustrated in Equation 1, glucose is oxidized to gluconic acid by the oxidized form of glucose oxidase (GO(ox)). It should be noted that GO(ox) may also be referred to as an “oxidized enzyme.” During the reaction in Equation 1, the oxidized enzyme GO(ox) is converted to its reduced state which is denoted as GO(red) (i.e., “reduced enzyme”). Next, the reduced enzyme GO(red) re-oxidized back to GO(ox) by reaction with Fe(CN)63− (referred to as either the oxidized mediator or ferricyanide) as illustrated in Equation 2. During the re-generation of GO(red) back to its oxidized state GO(ox), Fe(CN)63− is reduced to Fe(CN)64− (referred to as either reduced mediator or ferrocyanide).
When the reactions set forth above are conducted with a test voltage applied between two electrodes, a test current may be created by the electrochemical re-oxidation of the reduced mediator at the electrode surface. Thus, since, in an ideal environment, the amount of ferrocyanide created during the chemical reaction described above is directly proportional to the amount of glucose in the sample positioned between the electrodes, the test current generated would be proportional to the glucose content of the sample. A mediator, such as ferricyanide, is a compound that accepts electrons from an enzyme such as glucose oxidase and then donates the electrons to an electrode. As the concentration of glucose in the sample increases, the amount of reduced mediator formed also increases; hence, there is a direct relationship between the test current, resulting from the re-oxidation of reduced mediator, and glucose concentration. In particular, the transfer of electrons across the electrical interface results in the flow of a test current (2 moles of electrons for every mole of glucose that is oxidized). The test current resulting from the introduction of glucose may, therefore, be referred to as a glucose current.
Because it can be very important to know the concentration of glucose in blood, particularly in people with diabetes, test meters have been developed using the principals set forth above to enable the average person to sample and test their blood for determining their glucose concentration at any given time. The glucose current generated is detected by the test meter and converted into a glucose concentration reading using an algorithm that relates the test current to a glucose concentration via a simple mathematical formula. In general, the test meters work in conjunction with a disposable test strip that includes a sample receiving chamber and at least two electrodes disposed within the sample-receiving chamber in addition to the enzyme (e.g. glucose oxidase) and the mediator (e.g. ferricyanide). In use, the user pricks their finger or other convenient site to induce bleeding and introduces a blood sample to the sample receiving chamber, thus starting the chemical reaction set forth above.
In electrochemical terms, the function of the meter is two fold. Firstly, it provides a polarizing voltage (approximately 400 mV in the case of OneTouch® Ultra®) that polarizes the electrical interface and allows current flow at the carbon working electrode surface. Secondly, it measures the current that flows in the external circuit between the anode (working electrode) and the cathode (reference electrode). The test meter may, therefore be considered to be a simple electrochemical system that operates in a two-electrode mode although, in practice, third and, even fourth electrodes may be used to facilitate the measurement of glucose and/or perform other functions in the test meter.
As previously described, the amount of reduced mediator is measured at the working electrode through an oxidation current. The magnitude of the oxidation current is directly proportional to the working electrode surface area. Thus, in order to measure a glucose concentration in a precise and accurate manner, the working electrode area for a test strip must be reproducible and amenable to a robust manufacturing process. The ability to manufacture test strips with reproducible electrode areas becomes more difficult as the size of the working electrode area decreases. Because there is a market driven desire to reduce the volume of blood required to sufficiently fill a test strip, there is a need to manufacture test strips having a smaller working electrode area with high precision.
Test strips have often used an insulation layer to expose a pre-defined portion of the conductive layer, where the exposed portion is the effective working electrode area. Here, the effective working electrode area may be the area of the conductive layer capable of oxidizing a reduced mediator. The insulation layer may use an aperture or cutout to expose a portion of the conductive layer. One of the limitations of using an insulation layer may be that the aperture or cutout may not be sufficiently straight. Non-idealities of a straight edge may not significantly affect the working electrode area definition when the area is sufficiently large, but such non-idealities may become more of an issue as the working electrode area becomes smaller. As such, there is great interest in developing new methods for making test strips having a reproducible working electrode area that are robust and relatively inexpensive to implement.